A THF (2 mL) solution of 39 (71 mg, 0.14 mmol, 1 equiv.) was cooled to −78° C. in a 10-mL recovery flask, and a hexane solution of n-butyllithium (2.02 M, 141 μL, 0.28 mmol, 2.1 equiv.) was added dropwise over 5 min. After stirring the solution at −78° C. for 20 min, O-phenyl chlorothionoformate (39 μL, 0.28 mmol, 2.1 equiv.) was added in one portion. The resulting yellow solution was allowed to slowly warm to room temperature over 90 min and was subsequently quenched at room temperature with saturated aqueous NaHCO3. The mixture was extracted thrice with EtOAc. The organic extracts were combined, washed with H2O and brine, dried over Na2SO4, filtered, and concentrated in vacuo to an orange oil. Flash column chromatography (50 mL SiO2, 95:5 hexane:EtOAc) afforded 51 mg (81 μmol, 60% yield) of 40 as a colorless oil. TLC Rf 0.58 (7:3 hexane:EtOAc). 1H NMR (600 MHz; CDCl3): δ 7.41 (dd, J=8.4, 7.6 Hz, 2H), 7.30 (t, J=7.4 Hz, 1H), 7.08-7.06 (m, 2H), 5.74 (s, 1H), 5.53 (dd, J=11.5, 5.4 Hz, 1H), 4.99 (t, J=7.0 Hz, 1H), 3.80 (s, 3H), 3.25 (s, 1H), 2.56-2.53 (m, 2H), 2.44 (dd, J=14.6, 7.4 Hz, 1H), 1.86 (dd, J=12.9, 11.7 Hz, 1H), 1.69-1.65 (m, 7H), 1.58-1.56 (m, 1H), 1.47-1.41 (m, 2H), 1.37 (m, 2H), 1.23 (s, 3H), 1.23 (s, 3H), 1.03 (s, 3H), 0.95 (t, J=7.9 Hz, 9H), 0.58 (q, J=7.8 Hz, 6H). 13C NMR (126 MHz; CDCl3): δ 204.2, 194.6, 192.2, 177.0, 153.4, 134.9, 129.8, 126.9, 122.0, 118.6, 106.3, 84.6, 73.5, 69.8, 57.3, 55.7, 45.65, 45.58, 38.1, 34.3, 30.5, 29.7, 29.3, 26.1, 18.2, 17.8, 17.5, 7.4, 7.0. HRMS-ESI (m/z): [M+Na]+ calculated for C35H52O6SSi, 651.3152. found, 651.3130.
